1. Field of the Invention
The present invention relates to a laser drawing method and a laser drawing apparatus suitably adapted to laser annealing and other laser drawings.
2. Description of Related Art
Heretofore, for example in a liquid crystal display apparatus and other flat display apparatuses, a thin film transistor (TFT) has been used for a switching element. In the liquid crystal display apparatus, an active matrix method of forming a thin film transistor in a silicon film formed on a glass substrate has been put into practical use. In micro-fabrication of such a thin film transistor, for crystallizing an amorphous silicon thin film formed on a substrate, a laser annealing method using a laser light is used. By using the laser annealing method, it is possible to make a satisfactory thin silicon film transistor in a low temperature environment at about 500° C.
In the laser annealing method, a laser light emerged from a diffraction optical system using an acousto-optical diffraction element (hereinafter referred to as an AOD) is used. In the diffraction optical system using an AOD, an ultrasonic wave is generated in the AOD with a high frequency signal (hereinafter referred to as an RF signal) inputted to the AOD, and a laser light incident to the AOD is diffracted with the wave surface of the ultrasonic wave.
Generally, by changing the frequency of an RF signal inputted to an AOD while keeping the amplitude of the RF signal constant, the diffraction angle of a laser light incident to the AOD and is diffracted with the wave surface of an ultrasonic wave generated with the inputted RF signal is changed according to the change in the frequency of the RF signal. Further, the light intensity of a diffracted light changes depending on the diffraction angle of the diffracted light, because the diffraction efficiency (the light intensity of a diffracted light/the light intensity of an incident light to the AOD) changes depending on the diffraction angle.
Here, referring to FIG. 1, description is made with respect to an example of a change in the light intensity of a diffracted laser light according to related art. FIG. 1 illustrates a change in the light intensity of a diffracted light emerging from an AOD when the frequency of an RF signal inputted to the AOD is changed linearly in a cycle as time progresses and the amplitude of the RF signal is kept constant.
In FIG. 1, the frequency of the FR signal changes periodically in a saw-tooth wave state, and on the other hand, the amplitude of the RF signal is constant regardless of time. If such an RF signal is inputted to the AOD, the light intensity of an emerging diffracted light waves as illustrated in FIG. 1. The light intensity of the diffracted light takes a lower-limit value p1 at time t1 and an upper-limit value p0 at time t0. Similarly, the light intensity of the diffracted light takes an upper limit value p2 at time t2. Here, p0≅p2. Thus, even when the frequency of the RF signal increases from f0 to f1 and from f1 to f2 with the lapse of time, the light intensity of the diffracted light does not increase correspondingly, and changes between the lower limit value p1 and the upper limit value p2 as illustrated in FIG. 1.
When forming a thin film transistor using a laser annealing method, if the light intensity of a diffracted laser light is changed while the diffracted laser light is deflected to scan an object to be processed with an optical spot of the diffracted laser light, uneven exposure is caused on the object, resulting in deterioration of quality, which is undesirable.
In addition, the degree of a change in the diffraction angle of a diffracted light relative to a change in the frequency of an RF signal varies depending on the frequency of the RF signal, and even if the frequency of an RF signal is linearly changed, the diffraction angle of a diffracted light does not change linearly, that is, the diffraction angle of a diffracted light does not change at a constant speed.
Here, an example of a change in the diffraction angle of a laser light according to related art is described referring to FIG. 2. FIG. 2 illustrates a change in the diffraction angle of a diffracted light emerging from an AOD when the frequency of an RF signal inputted the AOD is changed linearly in a cycle with the lapse of time.
In FIG. 2 also, the frequency of the RF signal changes periodically in a saw-tooth wave state. It is understood from FIG. 2 that if such an RF signal is inputted to the AOD, as the frequency of the RF signal increases, the diffraction angle of a diffracted light also increases. Here, at time t1′, the frequency of the RF signal is f1′ and the diffraction angle takes a lower limit value θ1′. At time t2′, the frequency of the RF signal is f2′ and the diffraction angle takes an upper limit value θ2′. Thus, as the frequency of the RF signal linearly increases from f1′ to f2′ with the lapse of time, the diffraction angle of the diffracted light changes non-linearly between the lower limit value θ1′ and the upper limit value θ2′. That is, even when the frequency of an RF signal inputted to an AOD is changed linearly to diffract and thereby deflect a laser light incident to the AOD at a constant angular speed, the diffraction angle of a diffracted light is not changed at a constant speed (the angular speed of a diffracted light is not constant).
This can be explained as follows. When the propagation velocity of an RF signal in an AOD is “v” and the frequency of the RF signal is “f”, a distance “d” of a compressional wave formed in the AOD is expressed as d=v/f. A laser light incident to the wave surface of the compressional wave at an angle of θ1 is diffracted as a first order diffracted light in the direction of an angle of θ2 satisfying “d×sin θ1+d×sin θ2=λ”, wherein λ is the wave length of the laser light. Here, when θ1=θ2, that is, when 2d×sin θ1=λ, the diffraction efficiency (the light intensity of a diffracted light/the light intensity of an incident light to the AOD) becomes the maximum, and generally an AOD element is arranged such that the laser light is incident to the AOD element at an angle where the diffraction efficiency becomes the maximum. When the above-described formulas are modified and θ2 is expressed as a function of “f”, θ2=Sin−1(λf/v−sin θ1). That is, the diffraction angle θ2 of a diffracted light changes non-linearly relative to linear changing of the frequency of the RF signal. That is, the angular velocity of a diffracted light when the diffracted light is deflected is not constant.
If the diffraction angle of a diffracted laser light does not change at a constant speed, that is, if the angular velocity of a diffracted light when the diffracted light is deflected is not constant, the position of an optical spot of the diffracted light on an object to be processed is not moved at a constant speed and results in exposure unevenness on the object to be processed, which is undesirable.
A laser annealing apparatus irradiating a laser light to a substrate in a low oxygen density environment is disclosed in Japanese Unexamined Patent Application Publication No. 2004-87962.